1. Field of the Invention
This invention relates to an electronic volume device of a chopper type which controls the quantity of attenuation of an input analog signal to the device by switching the analog signal at predetermined periods and, more particularly, to an electronic volume device which is intended to miniaturize the device and to reduce the cost of the device by using a switching control unit which can be composed of a monolithic LSI (Large Scale Integration).
2. Description of the Related Art
Electric volume devices of a chopper type which have been proposed are shown in FIG. 5 and 6. In the device of FIG. 5, an input analog signal received at an input terminal 1 is turned on and off by an analog switch 2, the output of which is supplied via a low pass filter 3 for removing useless higher harmonics to an output terminal 4 from which the signal appears attenuated by a predetermined quantity. The switch 2 is turned on and off by a switching control signal input to the gate of the switch 2. Thus the duty cycle of the switching control signal is changed to change a quantity of attenuation of the signal output from the output terminal 4.
The switching control signal applied to the gate terminal of the analog switch 2 is formed by a volume control circuit 50. The volume control circuit 50 includes a volume control data generator 6 which generates a volume control data D.sub.s corresponding to a desired quantity of attenuation, a counter 8 which counts clock signals generated by a clock generation source 7 and having a predetermined frequency and which outputs a numerical data signal D.sub.x which changes periodically, and a comparator 9 which compares the volume control data D.sub.s output by the volume control data generator 6 and the numerical data D.sub.x output by the counter 8 and outputs, for example, a high level signal when D.sub.s &gt;D.sub.x and a low level signal when D.sub.s .ltoreq.D.sub.x. The output signal from the comparator 9 is applied to the gate of the analog switch 2 as the switching control signal.
In such arrangement, the period of the switching control signal output by the volume control circuit 50 is determined by the repeated counting Period of counter 8, and the duty cycle of the switching control signal is determined by the value of the volume control data D.sub.x set in the volume control data generator 6. When the volume control data D.sub.s set in the volume control data generator 6 of the volume control circuit 50 increases, the duty cycle of the switching control signal output by the volume control circuit 50 increases, so that the conductivity of the analog switch 2 increases to thereby reduce the quantity of attenuation of the input analog signal. In contrast, when the volume control data D.sub.s set in the volume control data generator 6 decreases, the duty cycle of the switching control signal output by the volume control circuit 50 decreases to thereby reduce the conductivity of the analog switch 2, and hence increase the quantity of attenuation of the input analog signal.
The structure of FIG. 6 employs a transistor 10 and a capacitor 12 instead of the analog switch 2 in FIG. 5. In this case, the switching control signal output by the comparator 9 of the volume control circuit 51 is applied to the base of the transistor 10 via an invertor 11, thereby controlling the quantity of attenuation of the analog signal as in the example of FIG. 5.
In such electronic volume device, the volume control circuit 50 or 51 can be fabricated as an integrated circuit. For example, it may be fabricated from a MOS (Metal Oxide Semiconductor) LSI (Large Scale Integration), a CMOS (Complementary Metal Oxide Semiconductor) gate array LSI or a bipolar type gate array LSI. Therefore, the volume control circuit of the electronic volume device can be miniaturized and made lighter. However, the circuit parts inclusive of the analog switch 2 shown in FIG. 5 or the transistor 10 shown in FIG. 6 as switching means is difficult to be constituted as a monolithic LSI, so that it requires many circuit parts, which is a big hindrance in the miniaturization, and weight and cost reduction of the electronic volume device. realizes miniaturization, and weight and cost reduction of the volume device.